The present invention relates to passive infrared motion detectors used for detection of motion of human targets moving in a spatial region monitored by the detector, by sensing far infrared radiation emanating from the targets. Such detectors contain an optical system, such as a reflective mirror device, which directs and focuses the infrared radiation from one or more detection fields-of-view or zones thereof. The detection fields-of-view or zones monitor the spatial region and pass through an infrared transparent window into the detector housing and onto one or more infrared radiation sensors, located within the motion detector.
The design of the optical system is usually based on the use of one or more optical elements, such as mirror segments, arranged in one or more rows, each row including one or more segments. The segments within the rows are arranged with their optical axes spread azimuthally in a plane, generally parallel to the horizontal, or inclined with respect to the horizontal.
Each of the segments is arranged to focus infrared energy emanating from a pre-defined detection zone onto an infrared radiation sensor such as a pyroelectric sensor, which is common to one or more segments. The combined detection zones of the multiple optical elements or segments, constitute the field-of-view of the detector, which is defined as the detection region covered by the detector or the “coverage” of the detector.
The mirrors are normally formed of plastic base such as Acrylonitrile Butadiene Styrene (ABS), preferably by injection molding or vacuum forming and are then coated with bright nickel or bright chrome as known in the art.
In such detectors, incoming infrared radiation enters the detector through an infrared transparent window in the detector housing and is reflected by the mirror segments to focus onto a pyroelectric sensor. The window is provided to prevent insects and other spurious matter from entering the detector.
A person moving through the field-of-view of the detector emits far infrared radiation having wavelength of 7-14 μm and causes the generation of a signal output from the infrared radiation sensor. This signal caused by a moving person is defined to be a “desired signal”. Signal processing circuitry of the detector detects and processes the desired signal and activates an alarm signal output when certain criteria are met.
Infrared motion sensors of the type described hereinabove are typically subjected to various sources of “undesired” radiation during their operation, such as radiation emitted by strong light sources. Furthermore, motion sensors using mirrored optics are generally unprotected from various undesired wavelengths of incoming radiation.
As a result, the “undesired” radiation reaches the mirror surface and is then reflected and focused onto the infra-red radiation sensor causing generation of “undesired signals”, which may cause false alarms and/or other inaccurate detection events.
Sunlight, as well as tungsten/halogen lamps, such as automobile headlights, produces one type of undesired radiation that is known to promote false alarms in infrared motion sensors. These radiation sources emit strong radiation in both the visible and the near infrared spectrum. Accordingly, compliance testing of infrared motion sensors in various countries often involves the use of a halogen light source at fairly intense levels (e.g., 2000 to 6000 lux) to determine the immunity of the motion sensor to this type of radiation.
Various solutions have been provided to minimize the effect of these undesired radiations.
The following published patent documents and other publications are believed to represent the current state of the art:
U.S. Pat. Nos. 3,949,259; 4,199,218; 4,245,217; 4,321,594; 4,342,987; 5,424,718; 5,712,622; 5,608,220 and 6,822,788.